The production of steel pipes for use in the oil and gas drilling industries has been the subject of a large amount of research and innovation. Apart from the materials and the means of forming a cylindrical tube, one aspect which has been a focus of attention is the coupling together of two separate pipes to provide a fluid-tight joint capable of withstanding both tensive and compressive forces.
The joints are normally formed by screwing together two pipes, each having complementary threads pre-cut into the ends of each of the pipes: addition of further pipes to the free end continuing, to build up a string. As an alternative but related method, a threaded coupling-sleeve is used to bridge across the ends of two pipes, but the principle remains the same.
To improve the sealing properties of the joint produced between pipes, and to give a seal capable of withstanding repeated handling under normal operating conditions, the profile and surface of the threaded part of the end of the pipes and the regions immediately around the threaded part towards the end of the pipe, are specifically designed to co-operate together to provide the seal. Typically the end of the pipe has a threaded portion cut either onto the outside of the pipe (to form a pin or male section) or into the inner surface of the pipe (to form a box or female section). The surface onto which the threaded portion is introduced can include a taper to assist the coupling process.
Alternatively the diameter of the pipe in the seal forming region can have been increased in comparison to that predominating along the length of the pipe, usually by cold forming, to allow a joint to be formed.
On the pipe, an unthreaded section is normally left between the end of the pipe and the threaded section, which unthreaded section is often referred to as a stop-shoulder. Particular emphasis has been placed in research on the stop-shoulder as this often constitutes the primary sealing region of the pipe joint. The stop-shoulder on the pin is usually profiled to engage a corresponding recess on the box section to form a strong seal. The particular profile is normally chosen to disperse efficiently the strain experienced in the pipe when torque is applied to form the joint and to retain the seal when the joint is in use. In addition to the profiling, coatings can also be applied to improve the fluid-tight nature of the seals.
However, as many reserves of oil and gas begin to run out and the price of oil climbs there is an increasing requirement and opportunity to extract oil and gas from reserves which would have hitherto been uneconomic. For example, extraction needs to take place from deeper or more inaccessible levels. Moreover, there is also a desire where possible to use existing bore-holes as a starting point to reach the more difficult deposits.
The outcome of this is that pipe joints need to be able to withstand high temperatures and pressures—both tensive and compressive—than has previously been the case. For example, many strings need to have bends of around 90° when going from a vertical orientation to a horizontal one. Such a bend obviously causes both compressive and tensive forces to act on the same joint.
One problem encountered on prior art joints is bending of the end of the pin on make-up of the joint. This can lead to galling: if not on the first time the joint is made up, then on subsequent make-ups. Although this problem can be addressed by increasing the tolerance on manufacture, this is an expensive solution and moreover does not completely remove the difficulties, particularly under normal working conditions.
It is an object of the present invention to seek to address the above problems.